Mammography is currently the gold standard for breast screening. However, the positive predictive value (fraction of cases identified as positive by mammography that truly are positive) is only 20 - 25%. Thus the vast majority of breast biopsies performed are negative. Biopsy is a very unpleasant experience for many, especially when it is a false positive. Researchers at the University of Virginia at Charlottesville (UVa) have developed a new diagnostic breast scanner by combining digital x-ray mammography and scintimammography, using a dedicated small field-of-view gamma camera, on a common gantry. In scintimammography, technetium-99m sestamibi is injected intravenously and concentrates preferentially at the site of malignant tumor. Early clinical evaluation of the dual modality scanner has shown promising results for more accurate diagnosis especially for small masses. The current dual modality scanner utilizes a mammography system, to which has been attached a parallel hole collimator and a gamma camera. Thus, each pixel in the gamma camera views a ray path perpendicular to the gamma detector surface, while each pixel in the x-ray detector views a ray path focused at the point source. This geometric discrepancy produces co-registration errors, especially for thick breasts and for masses lying near the top of the breast limiting the full benefit of the dual imaging diagnostic approach. The specific aims for this Phase I Small Business Innovation Program (SBIR) proposal are to (a) produce small focused collimators for the system currently at UVa, (b) to develop a microfabrication method using a sensitive, cost effective photoresist as the electroforming mold to fabricate the focused collimator, and (c) to evaluate co-registration using focused collimators on the UVa system. The specific aims for the Phase II SBIR are (a) to design an improved dual modality breast imaging system based on the experimental data of Phase I, (b) to fabricate a full-size mammography collimator appropriate for the new system, and (c) to test the dual modality breast imaging system incorporating the full-size collimator.